Fasteners are used in a variety of industries, including the aerospace industry and the automotive industry for attaching two or more workpieces to one another. These fasteners often include a nut plate and a nut which are part of an assembly. Prior art nut plates include riveted nut plates, glued-in-place nut plates and swedged nut plates. In riveted nut plates, two or more rivets are passed though pre-worked holes in the workpiece to secure the body of the nut plate to the workpiece. The rivets are often used in conjunction with a sealant or adhesive, which is placed between the workpiece surface and the nut plate. Riveted nut plate technology has numerous drawbacks. For example, additional time and effort is required to work each of the holes into which the rivets are placed and increased costs are incurred through use of additional materials (the rivets) and their machining.
Some “rivet-free” prior art nut plate technologies use adhesive instead of rivets to secure the nut plate to the workpiece. Drawbacks of this technology include the additional costs of adhesive, and the increased installation time needed to apply and cure the adhesive. Moreover, because achieving the proper strength of the bond between the nut plate and the workpiece is important, specialized skill and attention to the cure of the adhesive is required.
Swedged or expanded “rivetless” nut plate technology was developed to permit installation of the nut plates without rivets. In these designs, a sleeve or mandrel is installed and expanded or swedged into the workpiece, and is engaged with a base portion of the nut plate to hold it in place on the workpiece. Technologies in this design are disclosed in, for example, U.S. Pat. Nos. 4,732,518; 5,096,349; 5,245,743; 5,405,228; 7,059,816 and 7,114,900. Drawbacks to these technologies include the time and special tools required for installation, as well as the potential for damage to the workpiece substrate, particularly when the substrate is a composite material. For example, radial overexpansion of a work hole in a composite substrate may lead to delamination of the composite; conversely, insufficient radial expansion impedes proper shear load transfer.
There remains a need in the art for a nut plate technology that does not require the introduction of holes in the workpiece structure, and which allows for rapid, verifiably secure and less expensive installation of the nut plate.